1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and to an image forming apparatus and a process cartridge therefor using the electrophotographic photoreceptor.
2. Discussion of the Background
Recently, information processing systems using electrophotographic methods have noticeably been developed. Particularly, laser printers and digital copiers converting information to digital signals and optically recording the information have remarkably been improved in their printing qualities and reliabilities. These laser printers and digital copiers rapidly prevailing are required to print at higher speed and downsize as well as to produce higher quality images. Further, demands for full-color laser printers and full-color digital copiers capable of printing full-color images are rapidly increasing. At least four toner images having different colors each other need to be overlapped to form a full-color image, and therefore the full-color laser printers and full-color digital copiers are more seriously required to print at higher speed and downsize.
In order to make the apparatus print at higher speed and downsize, electrophotographic photoreceptors used therein need to have higher sensitivities and smaller diameters. Particularly, since a tandem apparatus effectively used for producing full-color images at high speed includes at least four photoreceptors, the photoreceptors are highly required to have small diameters. However, a photoreceptor having a smaller diameter naturally has less durability and needs to be replaced earlier. Therefore, the photoreceptor needs to have higher durability as well.
In order to make the photoreceptor have higher durability, images produced thereby need to have stable quality, and particularly, production of images having background fouling needs to be prevented. The background fouling is an image defect of innumerable microscopic spots developed on the blank area. It is conceivable that the background fouling is caused by the following phenomenon. When a photoreceptor is charged, a charge having the reverse polarity, which is induced at the electroconductive substrate, locally leaks and is injected into the photosensitive layer and further to the surface of the photoreceptor, and which becomes easily developable.
Although the background fouling is not actualized when the photoreceptor is new, the photoreceptor fatigue (poorly-charged photoreceptor) due to repeated use or the increase of the electric field intensity due to abrasion of the photosensitive layer actualize the background fouling which is one of large factors determining a life of the photoreceptor.
In order to prevent the background fouling, it is essential to prevent a charge from being injected into the photosensitive layer from the electroconductive substrate. As conventional technologies therefor, an undercoat layer or an intermediate layer is formed therebetween is disclosed.
For example, Japanese Laid-Open Patent Publication No. 47-6341 discloses an intermediate layer formed of a cellulose resin; Japanese Laid-Open Patent Publication No. 60-66258 discloses an intermediate layer formed of a nylon resin; Japanese Laid-Open Patent Publication No. 52-10138 discloses an intermediate layer formed of a maleic acid resin; and Japanese Laid-Open Patent Publication No. 58-105155 discloses an intermediate layer formed of a polyvinylalcohol resin.
However, these intermediate layers including a resin alone has high electric resistivities, and therefore the residual potential increases, resulting in occasional deterioration of image density and image gradation in negative and positive images.
In order to control the electric resistivity of an intermediate layer, methods of dispersing an electroconductive additive are disclosed.
For example, Japanese Laid-Open Patent Publication No. 51-65942 discloses an intermediate layer wherein a carbon-containing or a chalcogen-containing material is dispersed in a hardening resin; Japanese Laid-Open Patent Publication No. 52-82238 discloses a heat-polymerized intermediate layer formed by using an isocyanate hardener including a quaternary ammonium salt; Japanese Laid-Open Patent Publication No. 55-113045 discloses a resin intermediate layer including a resistivity regulator; and Japanese Laid-Open Patent Publication No. 58-93062 discloses a resin intermediate layer including an organic metallic compound. However, these resin intermediate layers alone occasionally have problems of producing moire images due to light interference in recent image forming apparatus using coherent light such as a laser beam.
In order to prevent the moire images from being produced as well as to control the electric resistivity of an intermediate layer, photoreceptors having an intermediate layer including a filler are disclosed.
For example, Japanese Laid-Open Patent Publication No. 58-58556 discloses a resin intermediate layer wherein aluminum oxide or tin oxide is dispersed; Japanese Laid-Open Patent Publication No. 60-111255 discloses an intermediate layer wherein an electroconductive particulate material is dispersed; Japanese Laid-Open Patent Publication No. 59-17557 discloses an intermediate layer wherein magnetite is dispersed; Japanese Laid-Open Patent Publication No. 60-32054 discloses a resin intermediate layer wherein titanium oxide and tin oxide are dispersed; and Japanese Laid-Open Patent Publications Nos. 64-68762, 64-68763, 64-73352, 64-73353, 1-118848 and 1-118849 disclose resin intermediate layers wherein powders of borides such as calcium, magnesium and aluminum; nitrides; fluorides; and oxides are dispersed.
The more the filler, the better to reduce the residual potential. The less the filler, the better to reduce the background fouling. Therefore, it is very difficult to reduce both of the residual potential and background fouling.
In order to solve this problem, a multilayered intermediate layer is disclosed. The multilayered intermediate layer is broadly classified to two types. One includes a resin layer wherein a filler is dispersed on an electroconductive substrate and a resin layer not including a filler on the resin layer wherein a filler is dispersed (FIG. 1A), and the other includes a resin layer not including a filler on an electroconductive substrate and a resin layer wherein a filler is dispersed on the resin layer not including a filler (FIG. 1B).
The former multilayered intermediate layer includes an electroconductive resin layer wherein a low-resistivity filler on an electroconductive substrate to hide defects thereof, and the above-mentioned resin layer is formed on the electroconductive resin layer. These are disclosed in, e.g., Japanese Laid-Open Patent Publications Nos. 58-95351, 59-93453, 4-170552, 6-208238, 6-222600, 8-184979, 9-43886, 9-190005 and 9-288367.
On the other hand, the latter multilayered intermediate layer includes the resin layer on an electroconductive substrate, and a resin layer wherein a low-resistivity or an electron-conductive filler is dispersed on the resin layer. These are disclosed in, e.g., Japanese Laid-Open Patent Publications Nos. 5-80572 and 6-19174.
However, the prevention of the background fouling and residual potential of the multilayered intermediate layer is occasionally more dependent on an environment.
Methods of including a polyamide resin in an undercoat layer and N-alkoxy(methoxy)methylated nylon in an undercoat layer or an intermediate layer are disclosed. For example, Japanese Laid-Open Patent Publication No. 9-265202 discloses a method of including an alkoxy methylated copolymer nylon resin having an alkoxy methylation of from 5 to 30% in an undercoat layer; Japanese Laid-Open Patent Publication No. 2002-107984 discloses a method of including an inorganic pigment and a crosslinked N-alkoxy methylated polyamide resin as a binder resin in an intermediate layer; Japanese Patent No. 2718044 discloses a method of including a N-alkoxy methylated polyamide resin, wherein each of Na, Ca and P atoms which are impurities has an elemental concentration not greater than 10 ppm in an undercoat layer; Japanese Patent No. 3086965 discloses a method of including a N-alkoxy methylated polyamide copolymer mainly constituted of a λ-amino-n-lauric acid in an intermediate layer; and Japanese Patent No. 3226110 discloses a method of including a polyamide resin including a unit having a structure in an intermediate layer. However, it is ineffective to prevent moire images from being produced even when a filler is dispersed in the undercoat layer or intermediate layer, and even a filler-dispersed layer occasionally increases the residual potential.
Various heat-hardening resins have been used as a binder resin in an intermediate layer, and some of the resins use a large amount of air contaminant formaldehyde which is one of causative agents of the sick house syndrome. They need a formaldehyde recycle system to prevent the formaldehyde from being discharged to the atmosphere. Therefore, it is essential to select a resin not discharging the formaldehyde, and a block isocyanate compound is effectively used in terms of this point.
For example, Japanese Laid-Open Patent Publication No. 61-163346 discloses a method of forming an intermediate layer including a urethane resin formed of a block isocyanate compound and an electroconductive powder, and optionally a second resin layer not including an electroconductive powder thereon. However, only the optional resin layer prevents a charge from being injected into the photosensitive layer from the electroconductive substrate, and therefore it cannot be said that this fully prevents the background fouling. In addition, Japanese Laid-Open Patent Publication No. 2004-177552 discloses a method of including an isocyanate compound together with a high-purity metal oxide and an oil-free alkyd resin. It cannot be said that this fully prevents the background fouling, either. Further, Japanese Laid-Open Patent Publication No. 3-33857 discloses a method of including a titanylphthalocyanine pigment having a specific crystal form in a photosensitive layer and a block isocyanate compound in an intermediate layer. However, this does not fully achieve the high durability, either.
Only the higher abrasion resistance of a photoreceptor is not sufficient enough to decrease the charge injection from an electroconductive substrate because the charge injection is increased by the charge deterioration of a photoreceptor due to the fatigue thereof, although decreasing an influence of the electric field intensity as time passes. Therefore, in order to prevent the background fouling due to repeated use of a photoreceptor and realize the higher durability thereof, the charge injection from an electroconductive substrate needs to be prevented and the abrasion of a photoreceptor due to repeated use needs to be decreased.
Conventional technologies to improve the abrasion resistance of a photoreceptor include, e.g., (i) Japanese Laid-Open Patent Publication No. 56-48637 wherein a hardening binder is used in a crosslinked charge transport layer, (ii) Japanese Laid-Open Patent Publication No. 64-1728 wherein a charge transport polymer material is used, and (iii) Japanese Laid-Open Patent Publication No. 4-281461 wherein an inorganic filler is dispersed in a crosslinked charge transport layer, etc.
However, (i) tends to increase residual potential and deteriorate image density because of poor compatibility of the hardening binder with a charge transport material, and impurities such as a polymerization initiator and an unreacted residue.
(ii) does not fully satisfy the durability required for an organic photoreceptor. In addition, a high-purity charge transport polymer material is difficult to obtain because it is difficult to polymerize and refine the materials. Further, a coating liquid including the charge transport polymer material occasionally has a high viscosity, resulting in a problem of preparing a photoreceptor.
(iii) also tends to increase residual potential and deteriorate image density due to a charge trap present on the surface of the inorganic filler.
In order to improve the abrasion and damage resistance of (i), Japanese Patent No. 3262488 discloses a photoreceptor including a hardened acrylate monomer. However, a three-dimensional network is not fully developed and the crosslinking density is low in the photoreceptor because a monomer is reacted therein including a polymer binder, resulting in insufficient abrasion resistance thereof.
As an abrasion resistance technology of a photosensitive layer in place of these technologies, Japanese Patent No. 3194392 discloses a method of forming a charge transport layer using a coating liquid formed from a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. The binder resin includes a binder resin having a carbon-carbon double bond and a reactivity with the charge transport material, and a binder resin having neither a carbon-carbon double bond nor a reactivity with the charge transport material.
However, a three-dimensional network is not fully developed, either and the crosslinking density is low in the photoreceptor, resulting in insufficient abrasion resistance thereof.
Japanese Laid-Open Patent Publication No. 2000-66425 discloses a photosensitive layer including a hardened positive hole transport compound having two or more chain polymerizing functional groups in the same molecule.
However, the photosensitive layer tends to have a surface crack or peel-off.
Because of these reasons, a need exists for a highly-durable photoreceptor producing images without background fouling.